apt-get source --build ignores MAKEFLAGS and CONCURRENCY_LEVEL

[Karl-Philipp Richter]

When I issue `apt-get build-dep grub-efi && CONCURRENCY_LEVEL=16 apt-get source --build grub-efi` I see from the CPU usage in comparison to usage of `make -j16` in the source directory that `apt-get` doesn't use the variable because it doesn't build in parallel.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: apt 1.0.1ubuntu2.5
ProcVersionSignature: Ubuntu 3.13.0-37.64-generic 3.13.11.7
Uname: Linux 3.13.0-37-generic x86_64
NonfreeKernelModules: zfs zunicode zavl zcommon znvpair
ApportVersion: 2.14.1-0ubuntu3.5
Architecture: amd64
Date: Sun Oct 26 16:51:02 2014
EcryptfsInUse: Yes
InstallationDate: Installed on 2014-10-23 (3 days ago)
InstallationMedia: Linux Mint 17 "Qiana" - Release amd64 20140530
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: apt
UpgradeStatus: No upgrade log present (probably fresh install)